Initial implementation of shared subscriptions

[andsel]

Release notes

Initial implementation of shared subscription subscribe and publish part.

What does this PR do?

Contains a series of updates to the broker internal structures to sustain shared subscription registering and dispatching of publish messages.

In CTrie's CNode add a separate shared subscriptions data structure to contains the shared subscriptions list related to share name for a given topic filter. It's implemented as a map keyed by share name which maps to a list of shared subscriptions.
Updated the CTrie's insertion method to use a data class SubscriptionRequest to capture the shared and non shared subscription actions.
Implemented test cases to cover the various MQTT spec requirements as integrations tests, and adapted utility classes (like the low level Client to permit the verification).

Why is it important/What is the impact to the user?

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files (and/or docker env variables)
• I have added tests that prove my fix is effective or that my feature works
• I have updated the Changelog if it's a feature or a fix that has to be reported

Author's Checklist

• complete simple shared subscription phase
• complete publish phase on shared subscription
• verify read access also for shared subscriptions, if the topic filter on shared subscription hasn't read permission for the client should it return a SUBACK with negative response? (understand it, and eventually write the integration test the verifies it).
• when a PUBLISH matches multiple overlapping shared subscriptions then one message per shared is generated.
• same client sending a new subscribe to same shared topic to update the QoS requested.

How to test this PR locally

Related issues

• Related [Meta] Implement Shared Subscriptions #791

Use cases